Complex older patients present substantial challenges to delivering medical care that is both guideline-based and patient-centered. There is little information available to guide a patient with complex health status and his or her physicians when they weigh the risks, burdens and benefits of a test or treatment plan to diagnose or treat chronic diseases. Despite the known relevance of treatment burden to patients'medical decision-making and quality of life in the settings of single diseases and at the end of life, there are no measures to assess treatment burden that can be used to improve patient-centered decision-making for patients with multiple chronic diseases. Treatment burden is defined in this application as the aggregate weight of the actions and resources patients and caregivers devote to health care management tasks, including effort, time, and out-of-pocket costs. Understanding the treatment burden experienced by complex patients is essential to delivering prioritized, patient-centered medical care. We propose to identify how complex health status is associated with treatment burden, operationalized as the presence and magnitude of self- reported difficulty with heath care management tasks, in multimorbid patients. We further investigate whether patient activation modifies this relationship, and, how treatment burden is related to health-related quality of life and quality of care using both cross-sectional and longitudinal data analyses. Specifically, we propose to use data from an ongoing study of older, multimorbid adults to investigate the following aims: 1) how different patterns of co-existing conditions are associated with treatment burden, 2) the association between presence and magnitude of treatment burden, and a) physical and mental-health related quality of life, and b) patient-reported quality of chronic illness care, and 3) the relationship between a) changes in patient activation and changes in treatment burden, and b) changes in treatment burden and changes in physical and mental health-related quality of life, and c) changes in treatment burden and changes in the ratings of patient-reported quality of chronic illness care over 18 months. If study hypotheses are substantiated, this study will lay the necessary ground work to develop an intervention consisting of assessing, reducing and managing treatment burden to improve patient-centered decision-making, which in turn should benefit adherence, physician-patient communication, health-related quality of life and quality of care.